Generally, a radial piston device usable as a motor or pump has the following elements: a circular casing with a bottom and a side wall and a top cover, which may be combined in one piece with the casing; an eccentric shaft journalled by bearings through the central part of the casing and the cover; a cylinder block, which may be machined in one piece with the casing, mounted directly on the eccentric shaft; the cylinder block having a number of cylinders, each fitted with a piston and radially arranged in the cylinder or block. During operation movement of the eccentric shaft drives the pistons to move reciprocatingly in the cylinders.
In this device, fluid or oil may be conducted via an oil duct, positioned in the eccentric shaft, and through oil distributing means to the space between the cylinders and pistons. With such an arrangement, when the pistons are driven to recirpocatingly move in the cylinders, the movement causes the intake and exhaust of fluid or oil, and the device is operated as a pump. However, when pressurized fluid or oil is transmitted from an outside fluid or oil source via the oil duct and oil distributing means to the spaces between the piston and cylinder, the pressure of the fluid or oil acting through the piston in combination with a connecting rod or acting directly upon the eccentric shaft produces a turning moment. If the eccentric shaft is fixed and stationary, a responding torque force causes the casing to rotate in the opposite direction. In such a case, the device is operated as a motor.
Many pumps and motors have been developed based on the above mentioned principles. One embodiment of such a radial piston device with a rotatable casing is described herein below.
On the eccentric of the eccentric shaft is rotatably fitted a star-like or pentagonal cylinder block with a plurality of hydraulic cylinders radially arranged in a plane perpendicular to the axis of the eccentric shaft. The block is enclosed with a circular casing having a bottom and a side wall with a plurality of planar surfaces evenly space around the inside of side wall and a top cover. Slidably fitted in each cylinder is a piston having a flat outer end engagingly held closely against the planar surfaces on the inside of the side wall of the casing by a coil spring or other resilient means. Arranged inside the eccentric shaft are two separate oil or fluid ducts for conducting a fluid into the cylinders.
During the operation of the device as a motor, one oil duct is connected to a high pressure fluid or oil source and the other to a low pressure fluid or oil source. In the center of the eccentric of the eccentric shaft there are two separate arcuate grooves each communicating respectively with one of the oil ducts in the eccentric shaft, so that the working fluid can be led to the cylinder and in turn act upon the casing through the piston. The resultant force exerted eccentrically on the casing produces a turning moment, and causes the casing to rotate.
Alternatively, if the device is connected to an external fluid source and the casing is driven by a power source, the planar surfaces on the inside of the casing cooperate with the action of the eccentric shaft to exert a force on the pistons causing them to move reciprocatingly in the cylinder. The reciprocating movement, with the aid of the oil ducts and oil distributing means results in the intake and exhaust of the fluid. The device is thus used as a pump.
The complete pump or motor is also fitted with oil leakage ducts, piston return springs, low friction bearings, thrust collars, sealing means, bolts etc.
Although the above-mentioned hydraulic pumps or motors have been gradually improved for many years, their structure and performance are still far from ideal. For most transmission systems, especially for the drive systems of vehicles, the working conditions such as, the required rotational speed and workload, vary over a wide range. However, a hydrostatic transmission system with a fixed displacement motor can only provide satisfactory performance over a narrow working range. Therefore, the application of hydrostatic transmission system is limited to low speed or low power rating systems and has not yet been used widely in vehicle drive systems, although it has many possible advantages; for example, the device may be provided with a completely continuously variable transmission or full automatic transmission with convenient layout. The reasons is that, until now, a simple and practical continuously variable displacement hydraulic motor has not been developed. Yet, to obtain high performance at most working conditions, the system must contain both a continuously variable displacement pump and a continuously variable displacement motor.
Moreover, the efficiency of a hydraulic pump and motor needs to be improved. In particular, the high efficiency area of these devices needs to be enlarged. Usually, a pump and motor now used only give high performance over a narrow range near its rated pressure and speed. If the working pressure and speed change over a wide range, the mean efficiency decreases significantly.
Furthermore, the rotational speed of a radial piston motor or pump needs to be increased. Generally, the working speed of a radial piston device ranges only up to about 200-300 rpm, its maximum speed being limited by mechanical factors and fluid mechanics. For instance, the mechanical efficiency of such a device decreases significantly with increasing rotational speed. Nevertheless, for most drive systems of vehicles and machines, the rotational speed required should be above 1000 rpm with widely varying working loads and working pressures. To obtain high efficiency over a wide range of working conditions, all of the factors which affect efficiency should be investigated thoroughly.
Additionally, cost is one of the key limitations to the practical application of a hydrostatic transmission system. This problem can be solved by simplifying the structure and technology, reducing the material requirements of hydraulic devices so that these become suitable for mass production.
To further broaden the applicability of such devices, the requirements for working fluid with specific properties and the need for filtration should not be strict. The device should be made less sensitive to load and temperature variations and vibration.
The best application for a radial piston device is as a wheel mounted motor for driving the wheel directly without need for additional speed change mechanisms. To accomplish this, the dimensions of the radial piston device, in particular its diameter, should be diminished and its weight reduced so that it can be conveniently mounted on the wheel and can withstand radial and axial shock load encountered while operating.
Based on the above considerations, the object of the present invention is to provide a hydraulic pump or motor which is simple in structure, with continuously variable displacement that is highly efficient over a wide rotational speed range.
Another object of the invention is to provide a hydraulic pump or motor having high mechanical and starting torque efficiencies by using hydrostatic support for the load sliding surfaces, thereby reducing friction losses.
Still another object of the invention is to provide a hydraulic pump or motor in which energy losses due to fluid flow in the oil ducts can be reduced, thereby enhancing the efficiency and maximum permissible working rotational speed.
A further object of the invention is to improve the volumetric efficiency of the radial piston device by optimizing the shapes and dimensions of its components to increase the flow resistance of the leakage fluid.
Another object of the invention is to provide a hydraulic pump or motor having good equilibrium performance with low PV ratings, i.e. pressure multiplied by velocity, for its load sliding surfaces under high speed and high pressure conditions.
Another object of the invention is to provide a hydraulic pump or motor, which can withstand heavy radial and axial loads, and which can be mounted directly on working components, such as a wheel, a sprocket, or a pulley. In this manner the transmission system of the machine, as a whole is further simplified.
Another object of the invention is to provide a wheel mounted motor which meets all of the above mentioned requirements. Using this type of wheel mounted motor as a part of a hydrostatic transmission system has many advantages and can replace transmission system presently used in many types of vehicles and machines.
A further object of the invention is to provide a low cost hydraulic pump or motor with simple structure and suitable for mass production.